The present invention relates to magneto-resistive memories, and more particularly, to the passivation of magneto-resistive bit structures.
Typical magneto-resistive memories such as giant magneto-resistive random access memory cells (GMR MRAM""s) use variations in the magnetization direction of a thin film of ferromagnetic material to represent and to store a binary state. Each thin film of ferromagnetic material in a GMR stack can be referred to as a magneto-resistive bit. During a write operation, the magnetization direction of a selected bit is set by passing an appropriate current near the bit, often using a word line, digital line, or sense line. The current produces a magnetic field that sets the magnetization direction of at least one of the layers in the ferromagnetic film in a desired direction. The magnetization direction dictates the magneto-resistance of the film. During a subsequent read operation, the magneto-resistance of the film can be read by passing a sense current through the bit structure via a sense line or the like.
Some prior art magneto-resistive bit structures are shown and described in U.S. Pat. No. 4,731,757 to Daughton et al. and U.S. Pat. No. 4,780,848 to Daughton et al., both of which are assigned to the assignee of the present invention and both of which are incorporated herein by reference. Illustrative processes for forming such magnetic bit structures are shown and described in U.S. Pat. No. 5,496,759 to Yue et al., and U.S. Pat. No. 5,569,617 to Yeh et al., both of which are assigned to the assignee of the present invention and both of which are incorporated herein by reference.
Such magneto-resistive memories are often conveniently provided on the surface of a monolithic integrated circuit to provide easy electrical interconnection between the bit structures and the memory operating circuitry on the monolithic integrated circuit. To provide a sense current through the bit structure, for example, the ends of the bit structure are typically connected to adjacent bit structures through a metal interconnect layer. The string of bit structures then forms a sense line, which is often controlled by operating circuitry located elsewhere on the monolithic integrated circuit.
A problem which arises as a result of the use of magneto-resistive memories is that conventional integrated circuit processes often cannot be used to form the contact holes or vias that are used to provide connections to the bit structure. For example, in a conventional integrated circuit process, vias are often formed by means of an etching process. First, a patterned photoresist layer, which defines the location and size of the vias, is provided over the integrated circuit. With the photoresist layer in place, vias are etched down to the bit structure. Once the vias are etched, a photoresist removal step typically is used to remove the photoresist layer.
In a typical integrated circuit back-end process the GMR bit ends are susceptible to damage by the corrosive chemicals used in the etching process. Furthermore, GMR bit ends may be exposed to a plasma environment with oxygen during removal of the photoresist in the oxygen asher process and are left unpassivated thereafter. Oxidation of the side walls of the bit ends can lead to significant degradation and adversely affect performance of the GMR MRAM""S. In order to avoid potential disastrous consequences, oxygen plasma photoresist removal is not generally utilized at the post Permalloy via etch or subsequent M3 etch stages. Instead, various solvent photoresist strip processes are utilized to remove the photoresist layers. Where a solvent or xe2x80x9cwetxe2x80x9d photoresist strip is used, it is necessary to choose the solvent with extreme care and to limit solvent use to mild solvents. In general, wet photoresist strips, although reducing the risk of oxidation, are prone to other defects and are not very production-worthy.
What would be desirable, therefore, is a magneto-resistive bit structure which is not subject to oxidation or corrosion by processing steps when forming vias. More specifically, it would be desirable to form a magneto-resistive bit structure without directly exposing the side walls of the bit ends to the potentially adverse effects of processes involved in via formation. This may allow more efficient and reliable back-end processing, which, in turn may reduce the defect density and increase the overall performance of devices incorporating magneto-resistive memories.
The present invention overcomes many of the disadvantages of the prior art by providing a magneto-resistive bit structure wherein the bits are protected from the potentially adverse effects of process steps that could damage the unprotected bit structure.
In one embodiment of the invention, magneto-resistive bits are encapsulated by means of an etch stop barrier material, such as CrSi, which is deposited as a barrier to cover both the top surface and side walls of a GMR stack. Encapsulation of GMR cells is highly desirable because exposed surfaces are susceptible to process damage resulting in degradation in the switching characteristics of the cells. The etch stop barrier material, preferably CrSi, provides a good barrier to oxygen and corrosive chemicals and thereby improves the robustness of GMR cells. It is selected to have a bulk resistivity which is low enough to allow sense line contact and high enough so that shunting will be negligible.
In another embodiment of the present invention, an etch stop barrier layer, for example, a layer of CrSi protects the top of the GMR stack while the side walls are protected by means of a dielectric extending along the edges of the GMR stack. For example, a SiN spacer can be formed by means of a controlled Permalloy via etch.
Metals deposited upon the passivated bit structure contact the bit ends only from the top through the etch stop layer, while the side walls of the bit ends are insulated from the metal by the dielectric spacer.
Passivation of magneto-resistive bit structures by the methods of this invention significantly improves the repeatability of GMR cells and makes the GMR back-end process more production-worthy.